Estimation of the filtration coefficient in intact dog lungs

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Abstract
The pulmonary capillary filtration coefficient (Kf) was estimated with a variety of methods. Most of these methods require estimates of or assumptions about lymph flow (Jl), tissue fluid hydrostatic pressure (Pt), and tissue fluid oncotic pressure (.pi.t). Kf was measured from the continuous recording of weight gain of intact dog lung lower left lobes resulting from increases in capillary hydrostatic pressure (Pc). Pc was increased in steps of .apprx. 5 mm Hg, and lung weight was allowed to become constant (or increase at a constant rate) before the next step. Once Pc exceeded a critical value (Pccritical), J1 (measured in 5 experiments) was constant even though the lobes continued to gain weight. Once Pc exceeded Pccritical the rate of weight gain (S) was constant, indicating that Pt was not changing. When Pc was further increased above Pccritical, lobes gained weight more rapidly. Kf was estimated from the relationship between S and Pc when Pc exceeded Pccritical (Kf = .DELTA.S/.DELTA.Pc). Pccritical was estimated as the extrapolated value of Pc at which S = 0. In 9 experiments Kf averaged 0.11 .+-. 0.06 (mean .+-. SD) ml/min.cntdot.mmHg-1 per 100 g estimated lung wet weight and Pccritical was 19.7 .+-. 5.4 mm Hg. There was a relationship between Pccritical and the plasma oncotic pressure (.pi.c): Pccritical = 1.55 .pi.c-6.8, r [correlation coefficient] = 0.79. The average value of .pi.c, measured on a membrane oncometer was 17.4 .+-. 2.2 mmHg before and 18.0 .+-. 2.4 mm Hg after Kf was measured in 8 experiments. These values were not significantly different (P > 0.25, paired data). Since there was little change in Jl, Pt, or .pi.c once Pc exceeded Pccritical, this method provides a good estimate of Kf in the intact dog lung.